A composition useful as a matrix for fiber-reinforced composites possesses a viscosity at 25.degree. C. of no more than 1000 mPas and provides a cured product displaying a breaking elongation of at least 5%. The composition comprises (A) at least one of a bisphenol epoxy resin (A1) and a phenol novolak epoxy resin (A2), a polyvalent epoxy resin formed from a mononuclear aromatic backbone (B), an aromatic amine curing agent (C), and a rubber (D).